The fundamental principle employed in the use of X-ray crystalography, or the calculation of a crystal’s structure is called Bragg diffraction. You may have noticed that when you look at the surface of a CD, you see a rainbow of colors. This is because when the lines on a reflective surface, such as a CD are close together, that is on the same order of magnitude of the wavelength of light, they reflect light in a way that is dependent upon the wavelength of the incident light, the incident angle, and the distance between the lines on the surface. As a result, the incident light is decomposed into it’s component wavelength’s or colors. For this reason, a scientific tool called a diffraction grating is at the heart of scientific equipment known as spectrometers. A spectrometer is used for identifying the properties of a light source based upon the spectral components of the emitted light. Mathamatically, this is called a Fourier Transform of the incomming signal, or light.
The principle we see in two dimensions on the surface of a CD also works in 3 dimensions. When we see a rainbow in the sky, it is the result of sunlight interacting with drops of water in the air. The light is diffracted in a manor that is a function of it’s wavelength, and as a result, all of the red light comes off of the particles at a slightly diffrent angle than that of green and blue.
The same diffraction principles what work for light also work for high energy light that we can’t see, such as X-rays. This difference is that in the case of X-rays, the wave length of the light is on the order of magnitude of the distance between different atoms in a molecule. A crystal is a solid material that has a regular structure, much like the diffraction grating we discussed earlier. In the case of an X-ray crystalography study, an X-ray is beamed through a crystal, and the resulting decomposition of the X-ray beam is captured on photographic film, or an X-ray sensor.
The mathamatical principle we mentioned earlier, the Fourier Transform works in two dimensions as well as one dimension. Using an inverse Fourier Transform technique, crystalographers can deduce the atomic structure of the crystal.
This technique has become one of the driving forces behind molecular biology as well as more common mineral crystals. Biologists work to purify proteins that they may have recovered or engineered, and if they are pure enough, there is a good chance that a crystal can be produced. If a protein crystal can be obtained, it’s exact three dimensional structure may be ascertained, and in fact, data bases of molecular structure have been created and are growing all of the time.
